1. Field of the Invention
The present invention relates to an optical switching element and to a device, an optically addressed type display medium and a display that are the optical switching element.
2. Description of the Related Art
In recent years, optically addressed spatial modulating devices, in which a photoconductive switching element and a display element are combined, have been developed. Such devices are being utilized as light bulbs for projectors or the like and are being investigated for potential uses in the field of optical information processing as disclosed in “Liquid Crystal Spatial Modulators and Information Processing”, (Liquid Crystal), Vol. 2, No. 1, 1998, pp. 3–18.
In the optically addressed spatial modulating device, the display element is driven to form a display image by causing the impedance of the photoconductive switching element to change depending on the amount of received light, and controlling a voltage applied to the display element, while applying a specific voltage to the element.
In particular, a medium, which writes by applying a voltage to a laminated optical photoconductive switching element and a display element having a memory function, while radiating an optical image, is being paid attention as an electronic paper medium, the medium of which can be separated from the writing device and carried.
Examples of display elements that have been investigated for such an optically addressed type medium include a liquid crystal display element such as that using a cholesteric liquid crystal or a ferroelectric liquid crystal, an electrophoretic element, an electrorotation element, a toner electrotransfer element, and an element in which any of these elements is encapsulated.
Examples of optical switching elements that have been investigated as optical switching element that can control voltage or current depending on the amount of received light include amorphous silicon elements that have been used in the field of electrophotography, OPC elements that have a function separation type double-layer structure with an organic photoconductor, and OPC elements that have a structure that includes a charge transporting layer (CTL) and charge generating layers (CGLs) provided on both sides of the CTL (hereinafter referred to as a dual CGL structure). In particular, the OPC element has the advantages of being able to be formed on a flexible substrate such as a PET film because it can be produced without heat treatment at high temperature, and able to be manufactured at low cost because it can be produced without a vacuum process. The dual CGL structure is particularly advantageous because it can be driven by alternating current and because, is used in combination with a liquid crystal element as the display element, an image is not burned into the display. Such a burn-in is caused by ion transfer by the bias component included in the applied voltage. The carrier used for driving may be positive or negative.
The OPC element with the dual CGL structure allows the display layer to be driven by alternating current, so that image sticking can be prevented, which would otherwise be caused by segregation of ions or the like. Therefore, high reliability can be achieved. As a result of analysis, however, it has been found that in a conventional charge transporting layer (CTL) comprising an charge transporting material (CTM) and a binder polymer in a ratio of 40:60, the CTM concentration is unevenly distributed in the thickness direction of the CTL and that there is a low CTM content layer (a CTM-poor layer) particularly in the vicinity of the surface of the known CTL.
This can cause the following problems. The charge-injection efficiency under the application of positive electric field to the CGL in the upper portion of the CTL can differ from that under the application of negative electric field. Therefore, the amount of the charge flowing under light conditions can vary, so that the electrical characteristics of the OPC element can be asymmetrical at the time of OPC illumination. Such a state can accumulate in the display layer causing a drift of the applied voltage and a degradation in the display characteristics. Moreover, the thickness of the CTM-poor layer increases with that of the CTL. Therefore, if the CTL is thick, the degree of the asymmetry can be high and the degradation in the display characteristics can be significant. Therefore, there are difficulties in increasing the thickness of the CTL, and the design of the balance with the impedance of the display layer is restricted. Thus, the OPC element cannot offer sufficient driving performance; the display characteristics are degraded; and the driving voltage margin is narrow.
Thus, the invention is provided to solve the above problems with the prior art and to achieve the following object. It is an object of the invention to provide an optical switching element having a dual CGL structure in which asymmetry of the electrical characteristics is suppressed so that the display characteristics or the driving performance can be improved, and to provide a device, an optically addressed type display medium and a display each using such an optical switching element.